Recently, reduction in size of a display device and accomplishment of high-definition display capability thereof are advanced together with advancement of cellular phones and information terminals. On the other hand, a display device which has a new added value is getting attention, such as a display device that allows a viewer to view different images depending on a position where the viewer watches the display device, i.e., a display device that provides images different from each other toward a plurality of view points, and a display device which produces a parallax image based on those images different from each other and which provides a stereoscopic image to the viewer.
A conventionally known scheme of providing images different from each other toward a plurality of view points synthesizes pieces of image data for respective view points, displays those pieces of image data on a display unit, separates the displayed synthetic images by optical separating unit including a lens, a barrier (a light blocking plate) with slits, and provides images to respective view points. The principle of image separation is based on restriction of pixels viewable depending on a view-point direction using the optical unit, such as a barrier with slits or a lens. Examples of image separating unit are a parallax barrier which is a barrier with multiple stripe-like slits, and a lenticular lens having cylindrical lenses which have a lens effect in a direction.
A stereoscopic display device having optical image separating unit is appropriate for mounting on a portable device since it does not need the use of a special eyeglass so that there is no burden of attaching the eyeglass. In practice, a portable device on which a stereoscopic display device including a liquid crystal panel and a parallax barrier is mounted is already available on the market.
According to the above-explained scheme, i.e., the display device that provides images different from each other toward a plurality of view points using optical separating unit, when the view-point position of a viewer moves and an image to be viewed is changed, a boundary between the image and another image appears darkly in some cases. This phenomenon originates from non-display regions (a light blocking unit, so-called a black matrix in general in the case of a liquid crystal panel) between a pixel and a pixel for view points being viewed. The above-explained phenomenon inherent to the movement of the view point of the viewer does not occur in the case of general display devices having no optical separating unit. Hence, the viewer may feel strangeness or reduction of the display quality from the above-explained phenomenon that occurs on a multi-view-point display device or a stereoscopic display device having the optical separating unit.
This phenomenon is called 3D moire in general. 3D moire is periodical varying of brightness (may be the varying of color in some cases) originating from different visions displayed on different angular directions. 3D moire is luminance angular fluctuation and does not become a problem depending on a view position. However, when fluctuation of brightness in the angular direction is large, undesirable effect for stereoscopic viewing may occur, so that it is desirable to set the brightness fluctuation to be equal to or smaller than a predetermined value.
Unexamined Japanese Patent Application KOKAI Publication No. 2005-208567 discloses a display device which has respective shapes and layouts of the pixel electrodes and light blocking unit of the display unit devised in order to overcome the problem originating from the optical separating unit and the light blocking unit, and which suppresses a reduction of the display quality.
FIG. 47 is a plan view showing the display unit of the display device disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2005-208567 (hereinafter, referred to as a Patent Literature 1). As shown in FIG. 47, an aperture 1075 is an opening of a sub pixel which is the minimum unit of an image display. As shown in FIG. 47, the layout of the apertures 1075 is defined by a vertical direction 1011 and a horizontal direction 1012. The shape of each aperture 1075 is hexagonal defined by a trapezoid symmetrical in the vertical direction 1011 and a rectangle having the same long side length as that of the bottom of the trapezoid which are arranged so that the bottom of the trapezoid and the long side of the rectangle contact with each other. Moreover, image separating unit comprises a lenticular lens having cylindrical lenses 1003a whose lengthwise direction is the vertical direction 1011 arranged in the horizontal direction 1012. The cylindrical lens 1003a has no lens effect in the lengthwise direction, but has the lens effect only in the short direction. That is, the lens effect acts on the horizontal direction 1012. Hence, lights emitted from the apertures 1075 of a sub pixel 1041 and of a sub pixel 1042 adjoining to each other in the horizontal direction 1012 are directed to directions different from each other.
The inclined side of the aperture 1075 is inclined in a different direction from the vertical direction 1011. A pair of sides having the same angle formed between the direction in which the inclined side runs and the vertical direction 1011 pass through the center of the aperture 1075, and are arranged so as to be axisymmetrical to a line parallel to the vertical direction 1011. Furthermore, the apertures 1075 adjoining to each other in the vertical direction 1011 are arranged so as to be axisymmetrical to a line running in the horizontal direction 1012. As a result, in the horizontal direction 1012, the position of the end of the aperture 1075 of the display panel and the position of the optical axis of the cylindrical lens 1003a differ from each other in the vertical direction 1011.
Hence, the aperture width in the vertical direction 1011 is substantially constant at the inclined portion regardless of the position in the horizontal direction 1012 when the apertures 1075 of the sub pixel 1041 and of the sub pixel 1042 are combined together.
That is, when it is presumed that a display-panel cut plane is present in the vertical direction 1011 that is orthogonal to the arrangement direction of the cylindrical lenses 1003a at an arbitrary point in the horizontal direction 1012, the display device of the Patent Literature 1 has the ratio of the light blocking portion (a wiring 1070 and the light blocking unit 1076) and the aperture substantially constant. Hence, when the viewer moves the view point in the horizontal direction 1012 that is the direction in which the images are separated, and the viewing direction changes, the ratio of the light blocking portion to be viewed is substantially constant. That is, the viewer does not occasionally view only the light blocking portion in a specific direction, and no display appears darkly. Accordingly, the display device of the Patent Literature 1 can suppress a reduction of the display quality originating from a light blocking region.
Moreover, Unexamined Japanese Patent Application KOKAI Publication No. 2009-98311 discloses a pixel structure suitable for the display device of the Patent Literature 1.
FIG. 46 shows a pixel disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2009-98311 (hereinafter, referred to as Patent Literature 2) and divided into four pieces. A gate line G and a storage capacitor line CS are formed on the same layer as that of a gate electrode of a pixel thin-film transistor 4TFT. Moreover, a storage capacitor 4CS is formed between a silicon layer 4SI and the storage capacitor line CS. As mentioned above, the silicon layer 4SI is connected to a data line D through a contact hole 4CONT, but another contact hole 4CONT provided in a pixel 4 other than the portion of the pixel thin-film transistor 4TFT is for connecting the silicon layer 4SI in the storage capacitor 4CS and a pixel electrode 4PIX.
The storage capacitor line CS is arranged in the extending direction of the gate line G, i.e., is connected to the storage capacitors 4CS of respective pixels adjoining to each other in the X axis direction. In respective pixels adjoining to each other in the X axis direction, positions of the pixel thin-film transistors 4TFT in the Y axis direction differ from each other, so that the storage capacitor line CS is bent and arranged in order to connect those transistors. Like the pixel thin-film transistor 4TFT, the storage capacitor 4CS is arranged at the upper-bottom side of a display region in a substantially trapezoidal shape in each pixel. Accordingly, the storage capacitor 4CS can be effectively arranged between upper-bottoms of respective pixels configuring an adjoining pixel pairs 4PAIR, thereby further improving the aperture ratio.
An intersection between the storage capacitor line CS and the data line D is arranged at a trapezoid inclining portion so that the storage capacitor line CS and the data line D are along with each other. It is preferable to reduce wirings arranged so as to be along the image separating direction as much as possible, and the above-explained display device has the data line D only. This further improves the image quality. This is because when the storage capacitor line CS is arranged along the image separating direction, the image of the storage capacitor line CS is enlarged by the image separating unit, resulting in a remarkable deterioration of the display quality. That is, the display device of the Patent Literature 2 has the gate line G and the storage capacitor line CS running in the image separating direction and formed on the same layer in order to suppress an image deterioration originating from the image separating unit and the storage capacitor line CS while reducing the number of processes.
Patent Literature 2 discloses a technique of forming a scanning line and a capacitor line through the same process in order to reduce the number of production process of the liquid crystal display device. In particular, there is a large demand of cost reduction for general small display devices, and it is desirable to configure a pixel array with the number of layers as small as possible.
Moreover, there is a demand for the display unit of the display device to increase the so-called aperture ratio which is defined by area ratio between the aperture contributing to the display brightness and the light blocking portion in order to make the pixel pitch finer so as to improve the high-definition display capacity and to improve the display brightness.
However, in order to accomplish the high-definition display of an image, a large number of pixels are arranged in a screen region which is originally small, so that it is necessary to make the size of a pixel finer. That is, how to reduce the pixel size is a technical issue. However, pixels with a finer size are almost realized together with the advancement of the microfabrication of semiconductor technologies.
As explained above, there is a tendency that pixels become finer, but it is not always enabled to reduce the size of electrical and electronic circuits, such as a switching device and an auxiliary capacitor for driving the liquid crystal in order to modulate light in proportion to the refinement of the pixel. This is because the switching device and the auxiliary capacitor are formed on a substrate like a semiconductor substrate or a glass substrate through the microfabrication technique, but there is a limit for realizable line width due to the limit of the semiconductor process. Moreover, even if finer process is technically possible, it results in the cost increase for a time from the standpoint of plant investment.
Liquid crystal display devices have a problem that because of the above-explained limit for refinement, a region where light is blocked increases, i.e., the aperture ratio decreases, and the light use efficiency of the whole display device decreases. There is a tradeoff relationship that when it is attempted to improve the image quality by refinement of the pixel, the light use efficiency decreases. Accordingly, there is a technical issue to realize a high-image-quality and highly efficient image display device and to realize a high-definition image simultaneously.
In particular, in the case of a small display device, because of the above-explained limit together with refinement, the ratio of wirings occupying the area of a pixel and that of a contact-hole area are extremely large, and the reduction of the aperture ratio is remarkable. It is necessary for the refined pixel to reduce the number of wirings in the pixel and that of the contact holes as much as possible.
Moreover, as is disclosed in “NIKKEI Electronics, Jan. 6, 2003, volume 838, p. 26 to 27” (hereinafter, referred to as a Non-patent Literature 1), recently, the applying field of the stereoscopic image display device and the application thereof become wide. As an example, a configuration in which image separation is performed in the direction in which the data line runs may be employed depending on the application of the display device. However, the inventor of the present invention found out that the high aperture ratio and the high image quality cannot be accomplished even if the pixel structure disclosed in Patent Literature 2 is designed as the above-explained configuration while maintaining the aperture shape and the light-blocking shape of the pixel disclosed in Patent Literature 1.
What the inventor of the present invention found will be explained below in more detail. As explained above, since the direction in which the gate line runs and the image separating direction are consistent according to the conventional technologies, the running direction of the storage capacitor line formed on the same layer as that of the gate line can be drawn in the same direction as the image separating direction so as not to interfere with the image separating unit. Likewise, when the pixel structure disclosed in Patent Literature 2 is applied to a display device that separates images in a direction in which the data line runs, it is necessary to draw the storage capacitor line formed of the same material as that of the data line in the image separating direction.
However, in order to protect the data line from any damage inherent to process conditions at the time of forming a switching device, in general, the data line is often formed in a process step after the formation of the gate line, i.e., on the substrate, the data line is formed on the upper layer of the gate line. As a result, it is necessary to form the storage capacitor line so that the capacitor is formed via an interlayer film having a small relative electric permittivity per unit area, and to set the area of the storage capacitor large. This results in an insufficient aperture ratio, and thus the transmissivity decreases.
Moreover, like Patent Literature 2, if the storage capacitor 4CS is formed between the silicon layer 4SI and the storage capacitor electrode formed of the same layer as that of the gate line, it becomes possible to form a capacitor via an interlayer film having a large relative electric permittivity per unit area, so that the area of the storage capacitor can be reduced. In this case, however, it is necessary to newly provide a contact hole that connects the storage capacitor electrode to the storage capacitor line, so that a sufficient pixel aperture ratio cannot be obtained, and thus the transmissivity decreases.
Moreover, according to the pixel structure of the display device disclosed in Patent Literature 2, the storage capacitor line on the same layer as that of the gate electrode is drawn so as to traverse the periphery of the switching device (TFT) in the image separating direction, so that the width in the Y axis direction of the light blocking portion located at the upper bottom of the substantially trapezoid becomes one that is obtained by adding the line width of the storage capacitor line and the line drawing space to the area of the TFT. The width of the upper bottom of the substantially trapezoid in the Y axis direction cannot be reduced without the change in a process rule, so that the width of the light blocking portion in the Y axis direction covering the upper bottom of the substantially trapezoid becomes large relative to the width of the aperture region in the Y axis direction in the case of pixels with a narrow pitch. When the image of the light blocking portion covering the upper bottom of the substantially trapezoid is enlarged by the image separating unit, it is visually recognized as a darkened spot or stripe on the display unit by the viewer, and thus the display quality decreases.
In this specification, the periodical varying of brightness (may be the varying of color in some cases), in particular, a luminance angular fluctuation originating from displaying of different images in different angular directions is defined as a “3D moire”. Moreover, a mixing of an image for another view point and leaking of an image to an image for a given view point are defined as “3D crosstalk”.
In general, a stripe pattern produced by an interference of structural objects having different periods is called a “moire stripe”. The moire stripe is an interference stripe produced depending on the periodicity of the structural object and the pitch thereof, and the 3D moire is a brightness varying produced due to the imaging characteristic of the image separating unit. Accordingly, the 3D moire and the moire stripe are distinguished in this specification.
The 3D moire does not become a problem depending on a view position, but when the fluctuation in brightness in the angular direction is large, an undesirable effect for stereoscopic viewing may occur, so that it is desirable to set the brightness fluctuation to be equal to or smaller than a predetermined value.